Autonomic Innervation

Question 1

What activities does the autonomic nervous system control?

Answer 1

It is responsible for controlling the activity of glands and organs and other activities in the body, which are really below the level of consciousness.

Question 2

How many divisions are there in the nervous system?

Answer 2

You can take the nervous system and divide it up into the central nervous system and the peripheral nervous system.

Question 3

What are the components of the central nervous system?

Answer 3

The Central Nervous System consists of the brain and the spinal cord.

Question 4

What are the components of the peripheral nervous system?

Answer 4

The Peripheral Nervous System consists of the 12 pairs of cranial nerves and the 31 pairs of spinal nerves.

Question 5

How can the peripheral nervous system be divided?

Answer 5

The peripheral nervous system can be divided into the sensory (or afferent) component and the motor (or efferent) component.

Question 6

How can the motor component of the peripheral nervous system be divided?

Answer 6

The motor component of the peripheral nervous system can be broken down into the somatic component - which is responsible for innervating skeletal muscle, and the autonomic component, which is responsible for innervating smooth muscle, cardiac muscle and glands.

Question 7

What is the basic difference between the somatic motor component to the nervous system and the autonomic motor component.

Answer 7

In the somatic motor component, the fundamental connection between the central nervous system and the effector organ, or a skeletal muscle cell, is a single neuron called a lower motor neuron.
On the other hand, in the autonomic system, there are two neurons involved in making the connection between the central nervous system and the effector organ. The first neuron is called the preganglionic neuron. The second neuron is called a postganglionic neuron.

Question 8

Where is the cell body of the lower motor neuron located?

Answer 8

It has its cell body located in the central nervous system (in the gray matter).

Question 9

Where does the lower motor neuron sends its fiber?

Answer 9

it sends its fiber out through the periphery to the effector, which is a skeletal muscle cell.

Question 10

Where is the cell body of the preganglionic neuron?

Answer 10

it has its cell body located within the gray matter of the central nervous system.

Question 11

Where the preganglionic neuron sends its fiber?

Answer 11

It sends its fiber - the preganglionic fiber- out to the periphery where it will enter a collection of nerve cell bodies in the periphery called a peripheral ganglion. And within that peripheral ganglion, the preganglionic fiber will synapse with a second neuron called the postganglionic neuron.

Question 12

Where the postgaglionic neuron sends its fiber?

Answer 12

The postganglionic neuron will send its fibers to innervate the effector organ: smooth muscle, cardiac muscle or a gland.

Question 13

How can the autonomic nervous system be divided?

Answer 13

The autonomic nervous system can be further broken down into sympathetic and parasympathetic components.

Question 14

What is the function of the sympathetic component of the autonomic nervous system?

Answer 14

It’s involved in mediating the body’s response to a stress: the so-called fight-or-flight response.

Question 15

What other name the autonomic sympathetic nervous system has?

Answer 15

The preganglionic neuron, the first neuron involved in the chain to the periphery, is restricted to the gray matter of the spinal cord, between the 1st thoracic (T1) and 2nd lumber (L2) cord segment. And so, sometimes the sympathetic system is referred to as the thoracolumbar outflow.

Question 16

In the upper two lumbar segments of the spinal cord, there’s an additional little horn of gray matter, where you find the cell bodies of the preganglionic neurons of the sympathetic system. What’s the name of that area?

Answer 16

Intermediate cell column or intermediate horn

Question 17

Describe the sympathetic innervation in spinal nerves C1 to C8 and L3 to Co

Answer 17

In order to get sympathetic innervation into those spinal nerves, the body will take advantage of the sympathetic chain fibers that are connecting the ganglia.
In this case, the preganglionic neuron with its cell body located somewhere between T1 and L2 of the spinal cord will send its fiber (the preganglionic fiber) through the ventral root. And that fiber will enter into the sympathetic chain by a white ramus communicans. And it can then ascend or descend the chain to a level where there is no outflow from the cord, but that requires sympathetic innervation.
For example: the fiber will ascend to a cervical sympathetic chain ganglion near one of the cervical spinal nerves. It can synapse there and the postganglionic fiber will enter into the cervical spinal nerve and then distribute with that cervical spinal nerve to the smooth muscle and arrector pili muscles and the sweat glands along the course of that nerve.
The preganglionic fiber doesn’t need to synapse at the level at which the postganglionic fiber will leave the sympathetic chain. This is just a variation on this pattern.
The Preganglionic fiber could leave the spinal cord through the ventral root, enter the spinal nerve and leave the spinal nerve via the white ramus communicans to connect with the sympathetic chain. And then, instead of beginning to ascend or descend, it could synapse within the first ganglion and the postganglionic fiber could descend through the chain and then leave through a gray ramus communicans to distribute along a lower spinal nerve. Or it could synapse and the fiber could ascend to the cervical ganglia and then leave the cervical ganglia via gray rami communicantes to enter into a cervical spinal nerve. (just variation on the pattern).
For this reason, it’s possible to note that between T1 and L2 spinal nerves there will be two rami communicantes connecting with the sympathetic chain. One is a white ramus communicans, bringing preganglionic fibers into the chain; the other is a gray ramus communicans taking postganglionic fibers out of the chain. But above spinal nerves T1 and below L2, you’ll only have a single ramus communicans connecting the sympathetic chain to the spinal nerve, and that’s a gray ramus communicans, which is taking postganglionic fibers from the chain and into the spinal nerve.

It’s important to notice that even for the spinal nerves C1 to C8 and L3 to Co, the preganglionic fiber may not ascend or descend; it may, in fact, synapse right within the first ganglia that it encounters. And the postganglionic fiber will ascend or descend before leaving the sympathetic chain through a gray ramus communicans.

Question 18

What is the prevertebal plexus?

Answer 18

A plexus of nerve fibers in front of the vertebral column

Question 19

What is he name of the chain of postganglionic neurons?

Answer 19

Sympathetic chain

Question 20

Does the prevertebral plexus have different names?

Answer 20

Yes.In different regions, there are different organs intervening between the plexus itself and the vertebral column behind. For example: In the abdomen, the aorta intervenes and so, it’s sometimes referred as the pre-aortic plexus in the abdomen.
It’s such an extensive network of nerve fibers interlacing over the surface of the anterior aspect of the vertebral column that it can be divided into regions. For example: In the abdomen it can be divided up into the celiac plexus around the origin of the celiac artery , beneath that you would find the aortic plexus and beneath the bifurcation of the aorta into the two common iliac arteries, that part of the prevertebral plexus is referred to as the superior hypogastric plexus. And it splits into the right and left inferior hypogastric plexuses by the presence of the organ of the pelvis.

Question 21

Describe the sympathetic innveration of organs of neck and thorax.

Answer 21

Sympathetic innervation to the organs in the neck and the thorax, involves preganglionic fibers that leave the upper segments of the spinal cord. They will synapse in an adjacent ganglion of the sympathetic chain and the postganglionic fiber will enter into this prevertebral plexus and from there, distribute to the organs that need to be innervated. For example: the heart.
Some of those preganglionic fibers will also leave the spinal cord from the upper thoracic segments and then ascend the sympathetic chain to one of the cervical ganglia, and then the postganglionic fiber will descend back down into the thorax - to the thoracic part of the prevertebral plexus. And then the fiber will distribute to one of the organs in the thorax.
Example: The preganglionic neurons have their cell bodies located in the intermediate horn of gray matter. They send their nerve fibers (the preganglionic fibers) through the ventral root into the spinal nerve. Then, they will leave the spinal nerve by a white ramus communicans. After that, they will synapse usually within the first ganglion that they encounter. And the postganglionic fiber will distribute in what;s called a cervical or cardiac branch into the prevertebral plexus. And from there, distribute to the organs.

Question 22

Describe the sympathetic innervation of organs of pelvis and abdomen.

Answer 22

The sympathetic innervation to the organs of the pelvis in the abdomen will be coming from segments of the spinal cord below T4. And typically what happens there is that the preganglionic fiber will enter into the sympathetic chain and then pass right through without synapsing. And they’ll follow a nerve down to the abdomen to the abdominal part of the prevertebral plexus; and they will synapse on a ganglion in the prevertebral plexus. And the postganglionic fiber will then follow one of the branches of the aorta to get to its target organ.
In the abdomen, the prevertebral plexus is situated in front of the vertebral column, but in this case, with the aorta intervening, the plexus is really on the surface of the aorta. And sometimes it is referred to as the preaortic plexus.
Because it’s so extensive, it’s divided up into regions like the celiac plexus (round the origin of the celiac artery), the aortic plexus (below the celiac plexus) and the superior hypogastric plexus that begins where where the aorta bifurcates and continues to go down in front of the vertebral column.
As it gets deeper into the plexus, it splits into two limbs: the right and left inferior hypogastric plexuses by the midline organs of the pelvis.
And within this preaortic plexus or prevertebral plexus, you find ganglia which contain postganglionic sympathetic neurons. They are named according to their location, like the celiac ganglia (around the origin of the celiac artery), the aorticorenal ganglion, the superior mesenteric ganglion (around the origin of the superior mesenteric artery) and so on.

How it happens: The preganglionic neurons have their cell bodies located in the intermediate horn or the lateral horn of gray matter in spinal cord segments from T5 and lower. The preganglionic fiber will leave the spinal cord through the ventral root of the spinal nerve. Then it will enter the sympathetic chain through a white ramus communicans and then pass through the sympathetic chain without synapsing and enter into a branch, which will carry it to the preaortic plexus or prevertebral plexus. And these branches that connect the sympathetic chain to the pre-vertebral or preaortic plexus are referred to as splanchnic nerves. So, the splanchnic nerve conducts these preganglionic sympathetic fibers to the prevertebral plexus in the abdomen and the pelvis. And there it will seek out a prevertebral ganglion and will synapse there. After that, the postganglionic fiber will follow along a blood vessel to get to the target organ in the abdomen or pelvis.

Question 23

What are the greater, lesser and least splanchnic nerves?

Answer 23

Looking at the thoracic part of the sympathetic chain, you can see branches coming from the thoracic ganglia. And they are all coming together to form one large splanchnic nerve called the greater splanchnic nerve, which will penetrate the diaphragm and enter the prevertebral plexus in the abdomen.
There’s another branch, called the lesser splanchnic nerve originating from the lower thoracic ganglia. And there is a still smaller one called the least splanchnic nerve, which comes from the last of the thoracic ganglia.

Question 24

Where the spanchnic nerves orginate?

Answer 24

Typically, the greater splanchnic nerve is made up of branches arising from the ganglia of the 5th, 6th, 7th, 8th and 9th thoracic ganglia. The lesser splanchnic nerve branches are coming from the 10th and 11th thoracic ganglia. And the least splanchnic nerve is arising from the 12th thoracic ganglia.

Question 25

Describe how the splanchnic nerves get to the target organs

Answer 25

The splanchnic nerves penetrate the diaphragm and enter into the prevertebral plexus in the abdomen, which is really situated on the surface of the aorta.
Entering the preaortic plexus, the greater, lesser and least splanchnic nerves are joined by splanchnic nerves that are coming from the lumbar sympathetic chain and even splanchnic nerves coming from the sacral sympathetic chain ganglia. They are all entering into this prevertebral plexus: they are seeking out ganglia within the plexus. Preganglionic fibers are synapsing there with the postganglionic fibers, which then follow the blood vessels out to the target organs.

Question 26

What is the visceral afferent component?

Answer 26

There’s an afferent or sensory component sometimes referred to as a visceral afferent component, that can be associated with these sympathetic nerve fibers that are going to the organs. These are just sensory neurons and they have to find some way to get back to the spinal cord, bringing sensation from the organ.

Question 27

Describe the route of the visceral afferent component

Answer 27

The pattern that they will follow is: from their sensory receptor in the organ, they will follow one of these splanchnic nerves up to the sympathetic chain, then they will leave the sympathetic chain with the white ramus communicans and enter the spinal nerve. After that, because they are sensory neurons, they will segregate out into the dorsal root (their cell bodies will be in the dorsal root ganglion) and they will send their central process into the Central Nervous System, where it will make connections within the central nervous system ( they are involved in reflex activities of the organs).
If the stimulus should reach the cerebral cortex, it’s not perceived typically as coming from an organ. But, it will be referred to an area of the body surface which is receiving its sensory input from the spinal nerve that carries these visceral afferents into the cord. So whatever spinal nerve arises from that cord level at which the visceral afferents are entering, that’s the area of the body surface to which pain will be referred from an organ that is being stimulated.

Question 28

What are the functions of the parasympathetic component of the autonomic nervous system?

Answer 28

It’s responsible for mediating functions that are more geared towards rest and restoration.

Question 29

Where are the preganglionic neurons of the parasympathetic component located?

Answer 29

The preganglionic neurons are located in certain nuclei of the brainstem associated with cranial nerves III, VII, IX and X, or in the gray matter of sacral cord segments 2,3 and 4. For that reason sometimes the parasympathetic component to the autonomic nervous system is referred to as the craniosacral outflow.

Question 30

Where are the postganglionic neurons of the parasympathetic component located?

Answer 30

The postganglionic neurons are in the walls or very close to the organs they innervate

Question 31

Describe the role of the vagus nerve in the innervation of thorax and abdomen

Answer 31

For cranial nerve X (or vagus nerve), the nerve arises from the brainstem and begins to descend through the neck and into the thorax. And these are preganglionic fibers. Branches of the vagus nerve will arise in the neck: they are called cervical or cardiac branches, which will descend to the prevertebral plexus in the thorax. Other branches will arise from the vagus nerve within the upper part of the thorax and enter into the prevertebral plexus. And those preganglionic fibers from both sites will then travel out to the target organ and synapse onto postganglionic fibers that are very close or, in fact, many times embedded right within the walls of the targeted organs. The vagus nerve will continue down through the thorax and through the diaphragm and into the abdominal part of the prevertebral plexus. And those vagal branches (preganglionic vagal branches) will follow the blood vessels out to the target organs and typically synapse on to the postganglionic parasympathetic fibers right within the walls of the target organ.
The vagus nerve will provide parasympathetic innervation through to the organs in the neck, the thorax and down into the abdomen as far as about the junction of the proximal 2/3 with the distal 1/3 of the transverse colon.
Also, the vagus nerve is actually supplying preganglionic vagal fibers to the organs supplied by the celiac artery and the superior mesenteric artery. The superior mesenteric artery supplies about the proximal 2/3 of the transverse colon and so that’s the limit of the vagal innervation.

Question 32

Describe the role of the sacral nerve S2, S3 and S4 in the innervation of thorax and abdomen

Answer 32

The rest of the transverse colon (the distal 1/3), the descending colon, and the organs in the pelvis receive their parasympathetic innervation from sacral cord segments S2, S3 and S4.

The preganglionic fibers will enter the sacral nerve S2, S3 and S4 and then branch out as what are called the nervi erigentes or the pelvic splanchnic nerves (not to be confused with sacral splanchnic nerves, which belong to the sympathetic system). And those preganglionic fibers will run out to the organs that they supply and synapse very close to or within the walls of those organs.

Question 33

Describe the route of the vagus nerve into the cardiac plexus

Answer 33

The vagus nerve is a cranial nerve, so it’s coming down from the brainstem, down though the neck, and it has a large component in it of preganglionic parasympathetic fibers. Some of those fibers are coming off in the neck as cardiac branches and some of them will come off in the thorax and they enter into the prevertebral plexus. And they are mingling with the sympathetic nerve fibers there. And those preganglionic fibers from those branches of the vagus nerve, will synapse onto postganglionic fibers which will then distribute to their target organs.
The vagus nerves when they enter into the thorax, they pass behind the lung root and join the esophagus. And the right and left vagus nerves join a plexus on the surface of the esophagus called the esophageal plexus. And that will carry them down into the abdomen.

When the vagus nerves get to esophagus, the fibers of the left and right vagus nerves will intermingle and form what are called the anterior and posterior vagal trunks. The anterior and posterior vagal trunks come down with the esophagus through the esophageal hiatus in the diaphragm. And those branches of those vagal nerve trunks will enter into the preaortic plexus. There, the preganglionic fibers will mingle with the sympathetic fibers in the preaortic plexus. Then the preganglionic) vagal fibers will follow the blood vessels out to the target organs. Different branches of the aorta have walls that are coated with nerve fibers. Some of them are postganglionic sympathetic fibers that are coming from their site of synapse in the ganglion. Others are preganglionic vagal fibers that are following along these blood vessels out to the target organs. And then they will synapse onto postganglionic fibers that are right within the walls at the target organs.

Question 34

Describe how the pelvic splanchnic nerves get parasympathetic innervation to the rest of the transverse colon, the descending colon and the organ of the pelvis.

Answer 34

The vagus nerve is going to look after everything down to the distal part of the transverse colon. And in order to get parasympathetic innervation to the rest of the transverse colon, the descending colon and the organ of the pelvis, that relies on the pelvic splanchnic nerves which are preganglionic parasympathetic fibers that are coming from sacral segments S2, S3 and S4. These fibers will simply join spinal nerves S2, S3 and S4. They will then leave as the pelvic splanchnic nerves and they will run into the lower part of the prevertebral plexus and run through it and follow the blood vessels out to the target organs where they will synapse onto postganglionic parasympathetic neurons that are embedded right within the walls of the target organs..

Question 35

Describe the Sympathetic innervation in spinal nerves T1 to L2

Answer 35

When you look at sections through the spinal cord at different levels, you see that the gray matter is distributed into a dorsal horn and a ventral horn, but when you get into the thoracic and the upper two lumbar segments of the spinal cord, there’s an additional little horn of gray matter called the intermediate cell column or intermediate horn and it’s there that you find the cell bodies of the preganglionic neurons of the sympathetic system.
Those preganglionic neurons have a number of options available to them to ensure that sympathetic innervation is distributed throughout the body. For example: From T1 to L2 the cord segments are distributed to the spinal nerves between T1 and L2. Arising from the spinal cord are the dorsal roots of the spinal nerve and the ventral roots of the spinal nerve. They come together to form the spinal nerve itself, which will then branch into its posterior primary ramus and larger anterior primary ramus.
Within the gray matter in the intermediate horn or the intermediate cell column, you find the cell bodies of the preganglionic sympathetic neurons. To get sympathetic innervation into spinal nerves, between T1 and L2, That preganglionic neuron cell body will send its cell body through the ventral root of the spinal nerve because this is essentially a motor system and motor neurons will pass through the ventral roots of spinal nerves. The nerve fiber will enter the spinal nerve and then leave it via a connection to the sympathetic chain called a white ramus communicans. And it will enter the sympathetic chain , and within the first ganglia that it encounters, it will synapse onto the postganglionic neuron of the sympathetic system. The postganglionic neuron will send its fiber back into the spinal nerve via what’s called a gray ramus communicans. And that fiber will distribute along that spinal nerve and its branches.
There will be postganglionic neurons going into the posterior primary ramus as well as the anterior primary ramus. And those postganglionic neurons will innervate smooth muscle cells along the course of that spinal nerve. For example: the smooth muscle cells in the walls of blood vessels where they’ll mediate vasoconstriction that will shunt blood away from the body surface and down towards the skeletal muscles. This will make sure that you’ll have lots of blood going to the skeletal muscles if you are going to run away. And there will be less blood toward the body surface, if you should be caught by your prey and cut, you won’t bleed as much.
That’s why people will often go ‘white with fear’ or ‘white with rage’, it’s because of the sympathetic response mediating the vasoconstriction of the blood vessels near the body surface and shunting blood down towards the interior.
Those fibers will innervate the smooth muscle cells or the smooth-muscle- like cells that are associated with the hair follicles (the arrector pili muscles) that makes your hair stand on end. Not such a factor for humans, but in animals like dogs and cats, when their hair is standing on end, they’re ready to fight.
The postganglionic fibers will also innervate the sweat glands along the course of that spinal nerve to make you sweat. That will help you to cool down if you’re fighting or running away. It will make it slippery in case you get caught by your prey: easier for you to get away.